Simrad SC90, Installation Manual

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About serial lines
A preferred method to establish communication between the SC90 and peripheral
devices, is by means of serial lines. Several serial line standards are available, each
with different qualities.
Unless otherwise specified, these cables must be provided by the installation shipyard.
RS-232 serial communication
An RS-232 serial port was once a standard feature of a personal computer, used for
connections to modems, printers, mice, data storage, uninterruptible power supplies,
and other peripheral devices. However, RS-232 is hampered by low transmission
speed, large voltage swing, and large standard connectors. [...] Nevertheless,
RS-232 devices are still used, especially in industrial machines, networking
equipment and scientific instruments.
https://en.wikipedia.org/wiki/RS-232 (September 2015)
Maximum length for an RS-232 serial cable is typically 60 meters with 2400 bps data
rate, however this depends on the cable quality. Always check the cable manufacturers
specifications for the actual "shunt capacitance". A common figure is 47.5 pF/m, which
gives a maximum cable length of about 50 meters.
Note
Most RS-232 cables do not support all the signals in the standard RS-232 specification.
Observe that long runs of unshielded cable will pick up noise easily. This is because
the RS-232 signals are not balanced.
Even though a standard exist for RS-232 pin configuration, certain manufacturer may
still choose their own connector pins for the various signals used. In order to make the
RS-232 connection to your peripheral device work properly, you must always consult the
relevant instructions provided by the device manufacturer.
For more information about the RS-232 serial communication standard, see (for
example):
• https://en.wikipedia.org/wiki/RS-232
RS-422 serial communication
While RS-232 is the most common serial interface for communicating with external
devices using the NMEA standard, it only allows for one transmitter and one receiver
on each line.
RS-422 provides a mechanism for transmitting data up to 10 Mbits/s. This interface
format uses a balanced signal on two wires. This increases both the maximum baud
rate and the physical length of the cable, and it reduces the noise. With a high quality
cable, you can use RS-422 on distances up to 1500 meters even in noisy environments.
RS-422 is also specified for multi-drop applications. This means that one transmitter can
send data to up to 10 receivers.
Simrad SC90 Installation Manual